1. Field of the Invention
This invention relates to catalytic conversion processes and apparatus, and more particularly, such processes and apparatus using plural reactors arranged in series.
2. Description of the Prior Art
Many catalytic conversion processes operate with several reactors rather than only one reactor. One reason for using multi-reactor systems is that often adequate process control cannot be maintained in a single vessel, especially in fixed bed adiabatic reactors. Moreover, it is often more economical to install several small vessels rather than one very large vessel. Additionally, process flexibility may be increased with multiple reactors so that different operating conditions can be used in each reactor, thus resulting in different product yields, variations in catalyst aging or ultimate life, changes in conversion of feed, or combinations of all the above. Reactor flexibility is particularly desirable in process plants which produce more than one product, as in the upgrading of lower olefins to gasoline and/or olefins. Such techniques are disclosed in U.S. Pat. No. 4,456,779 (Owen et al.) and generally known as an MOGD process.
Multiple reactors may be used in two basic flow configurations. Reactors may be manifolded to operate in parallel or series flow. However, reactors can only be practically used in parallel flow arrangement if the feedstock can be economically converted in a single pass through a catalyst bed. When the catalyst in the reactors requires frequent replacement or reactivation, an extra reactor may be installed and throughput can remain constant during catalyst replacement or reactivation.
Serial flow operation may be used when a series of partial conversion reactions take place across the catalyst beds, usually under endothermic or exothermic conditions. To attain the desired yield pattern, heating/cooling units are provided between reactors to perform inter-reactor heating cooling. An example of heating between reactors in series flow is the catalytic reforming of naphtha, an endothermic process. Furnaces are used between reactors to heat reactor effluent to the desired inlet temperature of the downstream reactor. In exothermic processes, reactor effluent may be cooled by using heat exchange or by direct cooling using gaseous or liquid quench streams. The catalyst beds may be in separate reactors or they may be placed in one large vessel with mechanical separation between the beds.
Some processes may use a combination of reactors in parallel flow configuration along with reactors in series flow, or the reverse combination. This is often done when two catalyst beds are used in a process. The initial catalyst may remove impurities or partially convert the feedstock, with the second catalyst completing the reaction. U.S. Pat. No. 3,998,899 discloses a fixed bed methanol-to-gasoline (MTG) process, wherein two catalysts are used in a process. In the MTG process, the first catalyst may be in one reactor or in two or more reactors operating in parallel flow with the effluent flowing in series to one or more reactors in parallel flow.
Another variation of series flow is used in a cyclic catalytic reforming process. Due to the need to reactivate the catalyst every few days, an extra, or swing reactor, is used to replace the reactor undergoing regeneration. Cyclic reformer operation with four reactors in series flow and one swing reactor as follows is known. Any reactor in flow position from one to four may be removed from service for regeneraton and be replaced by the swing reactor for process service. The plant piping allows the swing reactor to serve in any of the four process positions, and also undergo regeneration in the swing position. The process reactors always remain fixed in their process position unless they are in the regeneration position. In summary, each of the normal process reactors remain in a fixed process position except when it undergoes regeneration. The swing reactor temporarily replaces a process reactor when it undergoes regeneration.
In an olefins upgrading, such as oligomerization of lower olefins to produce gasoline and/or distillate range hydrocarbons, the most active catalyst is optimally located in the final process reactor position of a three reactor system operating in a series flow arrangement and the least active catalyst is optimally located in the first or initial feed reactor in order to attain the highest conversion of olefins to products. One or more intermediate reactors may assume process positions such that reactors with successively more active catalyst are in process positions progressively further downstream in the process sequence.